Mobile communications devices have become an integral part of society over the last two decades. Indeed, more than eighty-two percent of Americans own a mobile communications device, for example, a cell phone. The typical mobile communications device includes an antenna, and a transceiver coupled to the antenna. The transceiver and the antenna cooperate to transmit and receive communications signals.
The typical transceiver includes a power amplifier for amplifying low voltage signals for transmission via the antenna. Given that most mobile communications devices operate on limited battery power, energy efficient power amplifiers may be desirable. More specifically and as will be appreciated by those skilled in the art, Class C and E power amplifiers are common in mobile communications devices since they are efficient power amplifiers. These classes of power amplifiers are more efficient than Class A or B amplifiers, for example, but are subject to performance tradeoffs, for example, they may be nonlinear over certain frequencies and may introduce greater amounts of distortion into the amplified signal (if the signal requires a linear amplifier).
An approach to compensating for this tradeoff is to encode transmitted signals with constant envelope (CE) or continuous phase modulation (CPM) waveforms. These modulations provide for lower energy demands on the power amplifier of the transceiver, for example, by reducing the peak-to-average power ratio (PAPR), increasing average transmit power (providing greater transmission range), and increasing amplifier efficiency.
For example, U.S. Patent Application Publication No. 2008/0039024 to Ikeda et al. discloses an amplifying circuit. The amplifying circuit processes an input orthogonal frequency-division multiplexing (OFDM) signal and provides a pair of CE output signals to provide better power efficiency and less distortion.
A potential drawback to these modulations, for example, CPM or Minimum-shift keying (MSK), is the use of the inherent memory of the modulation when demodulating/decoding the waveform in order to obtain good demodulator performance. When the mobile communications device receives a transmitted signal which uses a modulation with memory, the decoder uses not only the current signal portion to demodulate but in addition uses previous signal portions (i.e. memory) to demodulate the current signal. In other words, the phase of the transmitted signal is not independent over successive signaling intervals. Decoding modulations with memory increases the computational and memory demands on the transceiver, i.e. a maximum likelihood sequence estimator is typically used to demodulate modulations with memory, thereby increasing the complexity of the device, which may be undesirable in a limited power compact mobile device.